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UCRL- 96157 .I PREPRINT

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I, L r' AUG 2 4 1987

High Resolution Seismic Attenuation Tomography

at Medicine Lake ,

John J. Zucca Paul W. Kasameyer

This paper was prepared for submittal to Geothermal Resources Council Annual Meeting Sparks, Nevada October 11-14, 1987

July 10, 1987

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High Resolution Seismic Attenuation Tomography at Medicine Lnke Vol-, Calif0n-h *

~~ - UCRL--96157 John 1 Zucca and Paul W..Kasameyer DE87 013790

Lawrence Livermore National Laboratory, Livemore, Califomia

SUMMARY

Medicine Lake Volcano, a broad about 5Okm east of in northern California. produced rhylotic eruptions as recently as 400 years ago. Because of this recent activity it is of considerable interest to producers of geothermal

energy. In a pint project sponsored by the Geothermal -...A - -e..--..____I. Research Program of the USGS and the Division of .&:- Geothermal and Hydropower Division of the US-DOE. the USGS and LLNL conducted an active seismic i.. experiment designed to explore the area beneath and arod the . The experiment consisted of eight explosions detonated in a 50 Ian radius circle around the volcano recorded on a 11 x 15 km grid of 140 seismographs. The travel time data from the experiment have been inverted for structure and are presented elsewhere in this volume. In this paper we present the results of an inversion for 1/Q structure using t* data in a modified AM inversion scheme. Although the data are noisy, we find that in general attenuative zones correlate with low velocity zones. In particular, we observe a high 1/Q zone roughly in the center of the caldera at 4 km depth in between two large recent flows. This zone could represent the still molten or partially molten source of the flows. Figure 1. Map showing location of Medicine Lake volcano Just east of the Cascades in northern California. The stars indicate the approximate locations of the eight INTRODUCTION explosions, which produced data recorded by 140 stations within the ellipse. The dotted lines indicate the For a number of years the USGS has used the refraction limes of Zucca, et. al., 1986. teleseismic travel-time residu